I actually use a cigarette paper wad over the flash hole. This just keeps powder out for shot to shot consistency. Some folks use a thick wad and a LP primer, with the wad making up the dimensional difference under the primer and softening the flame.
I mostly use .060 LDPE wads or .060 fiber wads, then a wad punched from notebook/printer paper in between it and the bullet base. You can see the paper wads floating down in front of you so you know you are getting separation.
Fred Cornell makes really nice press mounted wad punches, available from him or from Buffalo Arms. I use mine for punching paper only so it is easier to keep them sharp.